The present invention relates to the synthesis of phenol-functional or mercapto-functional resins and more particularly to a selective resin synthesis for flexibilizing such resins.
Vapor permeation curable coatings traditionally are a class of coatings formulated from aromatic hydroxyl-functional polymers and multi-isocyanate cross-linking agents wherein an applied film thereof is cured by exposure to a vaporous tertiary amine catalyst. More recently, the use of mercapto resins in vapor permeation curing of coatings is taught in commonly-assigned application U.S. Ser. No. 06/905,700 filed on Sept. 9, 1986, now U.S. Pat. No. 4,753,825. In order to contain and handle the vaporous tertiary amine catalyst economically and safely, curing chambers were developed. Curing chambers typically are substantially empty boxes through which a conveyor bearing the coated substrate passes and in which the vaporous tertiary amine, normally borne by an inert gas carrier, contacts such coated substrate. The use of aromatic hydroxy-functional polymers is recommended if an extended pot life system is required. If two-pack formulations are acceptable, then use of aliphatic hydroxyl-functional resins can be made. Multi-isocyanate cross-linking agents in traditional vapor permeation curable coatings contain at least some aromatic isocyanate groups in order for practical cure rates to be achieved.
Such traditional vapor permeation curable coatings requirements have been altered to a degree by the vaporous amine catalyst spray method disclosed by Blegen in U.S. Pat. No. 4,517,222. Such vaporous catalyst spray method relies on the concurrent generation of an atomizate of a coating composition and a carrier gas bearing a catalytic amount of a vaporous tertiary amine catalyst. Such generated atomizate and vaporous catalytic amine-bearing carrier gas flow are admixed and directed onto a substrate to form a film thereover. Curing is rapid and use of a curing chamber is not required. Moreover, all aliphatic isocyanate curing agents can be utilized in such spray process. Aromatic hydroxyl groups on the resin, however, still are required.
One drawback to the requirement of aromatic hydroxyl groups on the resin is the inherent limitation which such aromaticity provides in formulating high solids coatings. The same is true of the requirement of aromaticity in the multi-isocyanate cross-linking agent. Such non-volatile solids content restriction even applies to the vaporous amine catalyst spray method described above.
Yet, despite the foregoing limitations which arise by virtue of the use of phenolic hydroxyl groups, aliphatic hydroxyl groups are not sufficiently responsive to vapor permeation cure to permit early film development. That is, a prime advantage of vapor permeation curable coatings is that they rapidly develop very early film properties so that the coated part can be handled on the coatings line without fear of damage to the coating. Over the long term, aliphatic hydroxyl groups will fully cure with the polyisocyanate cross-linking agents, through extended cure times means that early handling of the coated part is lost.
With respect to phenolic functional resins and mercapto-functional resins, the requisite phenol or mercapto functionality most frequently is introduced into the resin by coating techniques involving the reaction of a carboxyl-functional capping agent with hydroxyl or equivalent functionality on the resin backbone. Such capping synthesis predominates in the art since most phenol/mercapto capping agents are readily synthesized to contain carboxyl functionality. The introduction of different functionalities into the phenol/mercapto capping agent can cause difficulties in stability or later reaction of such capping agents. U.S. Pat. Nos. 4,331,782 and 4,366,193 form adducts from carboxyl-functional capping agents which adducts contain both phenol functionality and aliphatic hydroxyl functionality. The use of such capping agent adducts still would require functionality on the resin reactive with aliphatic hydroxyl groups, which functionality most typically is carboxyl in nature.
Some coatings applications require flexibility from the resin, e.g. the coating of flexible plastic substrates such as reaction injection molding (RIM) urethanes or the like. One method for flexibilizing polyester or like resins involves their chain extension with polyisocyanates. The introduction of urethane linkages into the polymer backbone allows the formulation of more flexible coatings with the same cross-link density compared to a comparable polymer of the same molecular weight. The introduction of urethane linkages into the polymer backbone, though, is not a task without problems. For example, British Pat. No. 1,351,888 calls for the formation of a hydroxy urethane resin formed by the reaction of an excess of polyol with polyisocyanate, which hydroxy urethane resin then is reacted with a phenolaldehyde condensate (capping agent). The problem with such a proposal is that the capping is run after the isocyanate chain extension step. Side reactions and/or degradation of the resin are real risks in such a synthesis technique. In addition, one is limited to the use of phenol-aldehyde condensate capping agents. If phenol or mercapto functional carboxylic acid capping agents are used to cap a hydroxy urethane resin, problems also are encountered. The capping step is run at a high temperature (180.degree.-220.degree. C.) for extended periods of time (4-12 hours). These severe reaction conditions are deterimental to the stability of the urethane linkages introduced into the backbone. Accordingly, a more general and useful synthesis technique would be useful in formulating isocyanate-flexibilized phenol/mercapto resins.